Create features which contribute socket-factory dependencies for Google Cloud SQL

[sdelamo]

Create features for the different support databases

gcp-cloudsql-mysql
gcp-cloudsql-postgresql

A feature is a class annotated with @Singleton which implements Feature. It should be placed in starter-core. Tests should use the BuildTestVerifier API.

You can contribute a dependency by using GeneratorContext::addDependency.

Since those dependencies versions are not in the Bill of Materials (BOM), you will need to define them here:

https://github.com/micronaut-projects/micronaut-starter/blob/3.9.x/gradle/templates.versions.toml

Search for examples with lookupArtifactId about how to add a dependency which reads its version from that file .

The base of the PR should be 3.10.x